The use of sensors to detect and measure temperature, gas concentrations and the like is well established in the art. Sensors are often positioned at remote or dangerous locations, such as in a mine or at the outlet of a smokestack, and the information measured by the sensors must be transmitted long distances, often up to one mile or more, to current meters. The signal produced by such a sensor is an analog signal covering a continuum of states and a standard has developed in the industry, as set by the Instrumentation Society of America, in which a 20 milliampere signal is transmitted for a full scale sensor reading, a 4 milliampere signal is transmitted for a zero sensor reading, and a proportional signal between 4 and 20 milliamperes is transmitted for a sensor reading between zero and full scale.
A variety of 4-20 milliampere transmitters are known in the art, such as the Burr-Brown XTR110 and the Analog Devices AD693. These transmitters typically include an instrumentation amplifier that references its output to the potential across the load and generates a measurable current related to the voltage developed by the sensors. The prior art designs require either a high voltage power source, or both a positive and negative power source to operate. The use of two power supplies is an obvious disadvantage and the use of a higher voltage is dangerous in certain environments.
It is, accordingly, an object of the present invention to provide a 4-20 milliampere transmitter which operates on a single, relatively low voltage power source.
Since the wires between the sensor and the current meter are often lengthy, it is desirable if the measured current drops to zero when a break in any wire occurs. If the sensor requires power from a remote source, three wire systems are used. These systems include a positive voltage wire, a current output wire, and a common wire. Prior art systems do not satisfactorily indicate a breakage of the common wire.
It is a further object of the present invention to provide a 4-20 milliampere transmitter which has a zero output current when any of the three conducting wires, and particularly the common wire, breaks.
It is desired to accomplish these objectives with a circuit that is suited to handling voltage transients and in which the number of components, including amplifiers, is kept to a minimum.